Process for the preparation of blue-flourescence emitting carbon dots (CDTS) from sub-bituminous tertiary high sulfur Indian coals

Abstract

The present invention relates to a process for the preparation of blue-fluorescence emitting carbon dots (CDTs) from sub-bituminous tertiary high sulfur Indian coals. More particularly, the present invention relates to the production of characteristics carbon dots from low-quality Indian coals by an ultrasonic-assisted wet-chemical method. Also, the present invention provides a simple and environmentally benign method for fabrication of characteristics and size-controlled carbon dots.

Claims

1. A process for the preparation of blue-fluorescence emitting carbon dots (CDTs) from a carbon source, wherein the carbon source is sub-bituminous tertiary high sulfur coals, wherein the process comprising the steps of: a) pulverizing the sub-bituminous tertiary high sulfur coals to 72 BS or 70 ASTM mesh size (0.211 mm) particles; b) mixing the pulverized coal obtained in step (a) with 20-30% of an oxidant under ice cold condition, wherein the oxidant is hydrogen peroxide; c) sonicating the mixture obtained in step (b) at room temperature for 6 hours to obtain a brown-red solution and cooling it in an ice-water bath followed by slow pouring into a beaker containing 500 ml of crushed ice; d) neutralizing the mixture obtained in step (c) by adding ammonia solution dropwise until pH 7 is attained; e) filtering the neutralized mixture obtained in step (d) through 0.22-m polytetrafluoroethylene membrane; f) dialyzing the filtrate obtained in step (e) in 1 kDa dialysis bag against ultrapure water for 5 days and collecting the solutions; g) concentrating the solutions obtained in step (f) using rotary evaporation to obtain the desired carbon dots and storing under ice-cold condition.

2. The method as claimed in claim 1, wherein the carbon source is selected from sub-bituminous tertiary high sulfur coal having different percentages of carbon.

3. The method as claimed in claim 1, wherein the carbon source comprises low-quality coal.

4. The method as claimed in claim 1, wherein the sonication frequency is 20-40kz and output power is 700-1000W in a microprocessor based bench type ultrasonic bath.

5. The method as claimed in claim 1, wherein the formed carbon dots have diameter ranging from 1-6 nm.

6. The method as claimed in claim 1, wherein the formed carbon dots are crystalline and functionalized with a plurality of functional groups selected from amorphous carbon, oxygen group, crystal group and carboxyl group.

7. The method as claimed in claim 1, wherein the formed carbon dots comprise carbon quantum dots as well as graphene quantum dots.

8. The method as claimed in claim 1, wherein the formed carbon dots emit blue fluorescence.

9. The method as claimed in claim 1, wherein the sub-bituminous tertiary high sulfur coal is from India.

10. The method as claimed in claim 1, wherein the formed carbon dots have diameter ranging from 2-5 nm.

11. The method as claimed in claim 1, wherein the formed carbon dots have diameter ranging from 10-30 nm.

12. The method as claimed in claim 1, wherein the formed carbon dots have diameter ranging from 1-4 nm.

13. The blue-fluorescence emitting carbon dots obtained by the process as claimed in claim 1 useful in optical-imaging and bio-sensing including road stickers, road signs, paints, photographic processing materials and the like.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The present invention provides a method of making carbon dots with novel characteristics from a carbon source, such as low-quality Indian coals. The method of the present invention involves: selecting the carbon sources, exposing the carbon sources to an oxidant to form carbon dots, and separating the formed carbon dots from the oxidant.

(2) Coal Sources:

(3) Various types of carbon sources may be utilized to form carbon dots. In some embodiments, the carbon source is Tertiary high sulphur coal. Coal is the most affordable, abundant and readily combustible energy resources being used worldwide. The chemical structure of coal is complex. The simplified composition contains angstrom or nanometer sized crystalline carbon domains with defects that are linked by aliphatic amorphous carbon. Although research on the nano-chemistry of coal has been initiated, but their practical application in electrical, mechanical and optical field is found to sparse. Consequently, coal is still mainly used as an energy source.

(4) The structural characteristics of the coals create a perception that coal is only useful for producing energy through burning. The carbon-rich natural resource coal needs no longer to be burned only for the purpose of generating electricity but can be used as a feedstock to fabricate carbon material including carbon nanomaterials. Approximately one-third of the coal produced in the world, except for China, is low-quality. As a result, there is growing importance and demand for the utilization of low-quality coal. The low-quality coal needs to be cleaned before utilization. In this context, the different aspects of value addition to low-quality coals are very much essential. The inventors of the present invention have utilized low-quality coal for the first time to produce carbon dots. Various types of coals may be utilized as carbon source to form carbon dots. In some embodiments, the carbon source is sub-bituminous type coal of Tertiary age collected from Northeast region (NER) of India. In some embodiments, unique coal structure has advantages over pure sp.sup.2 carbon allotropes for producing carbon dots. The use of additional carbon sources can also be envisioned.

(5) Exposing the Carbon Sources to an Eco-Friendly Oxidant:

(6) In some embodiments, carbon dots form by exposing the carbon source to an environment friendly oxidant. Various oxidants may be utilized to form carbon dots. In some embodiments, the oxidant includes Hydrogen Peroxide (20-30%), which is environment friendly. The utilization of hydrogen peroxide at different concentration can also be envisioned. The utilization of additional oxidant can also be envisioned.

(7) Various methods may be utilized to expose carbon source to oxidants. The exposing occurs while the carbon source and the oxidant are in a liquid solution. In some embodiments, the exposing includes sonicating the carbon source in the presence of the oxidant. In some embodiments, the exposing includes stirring the carbon source in the presence of the oxidant. In some embodiments, the sonicating occurs for 6 hrs. In some embodiments, the sonication time may increase or decrease depending upon the carbon source. In some embodiments, the oxidant may be exposed to the carbon source in a slow manner. For instance, in some embodiments, the oxidant is mixed dropwise with the carbon source under ice cold conditions. Additional methods of exposition of carbon sources to oxidants can also be envisioned.

(8) The exposure of carbon sources to oxidants followed by sonication can lead to the formation of carbon dots. Without being formed by the theory, inventors envision that upon the exposure of coal to oxidants followed by sonication, carbon dots form by exfoliation of the carbon source. In particular, inventors envision that the crystalline carbon within the coal structure was oxidatively displaced to form carbon dots.

(9) Separation of Carbon Dots:

(10) The method of the present invention includes a step of separating the formed carbon dots from the oxidants such as H.sub.2O.sub.2. Separating includes neutralizing a solution that contains the formed carbon dots by ammonia solution, filtering the solution and dialysis of the solution. In some embodiments, the separating steps include dialysis of the solution that contains the formed carbon dots. Additional methods of separating carbon quantum dots from oxidants can also be envisioned.

EXAMPLES

(11) The following examples are given by way of illustration only and therefore should not be construed to limit the scope of the present invention in any manner.

Example 1: Fabrication of Carbon Dots from Tertiary Indian Coal (Sub-Bituminous Rank)

(12) In this example, there is provided a facile approach for producing carbon dots from sub-bituminous rank coal. The synthesized carbon dots from coal in a cost-effective manner are water soluble and fluorescent in aqueous solution. In this example, an inexpensive facile one-step wet-chemistry route was used to synthesize/produce carbon dots from carbon sources of four different types of sub-bituminous rank Northeast Indian coals: Tirap-60, Tirap-20, Coal-NK, and coal-NG. The physico-chemical characteristics of the coal samples used in the investigation are shown in Table 1.

(13) TABLE-US-00001 TABLE 1 Physico-chemical characterization of the coal samples (as received basis, wt %) SL Coal Proximate analysis (%) Ultimate analysis (%) TS No Samples M Ash VM FC C H (%) 1. TD-T60 2.20 2.95 45.02 49.82 80.40 5.97 3.62 2. TD-T20 2.35 2.33 50.27 45.05 80.90 8.19 1.90 3. TD-NK 3.82 19.04 36.32 40.83 61.20 5.86 3.26 4. TD-NG 9.11 4.44 49.07 37.38 67.50 7.31 3.59 M: Moisture; Ash: Ash content; VM: Volatile Matter; FC: Fixed Carbon; C: Carbon; H: hydrogen; N: Nitrogen; TS: Total Sulphur

(14) The specific methodologies used for the synthesis of carbon dots are summarized herein:

Example 1.1 Synthesis/Production of Carbon Dots from Coal Sample (Tirap-60)

(15) 12 g of coal sample was mixed with 200 ml hydrogen peroxide (20-30%) under ice-cold condition. The reaction mixture was then sonicated (frequency: 20-40 kz, output power: 700-1000 W) in a microprocessor-based bench type ultrasonic bath for 3-6 hrs at atmospheric pressure and temperature. The solution was cooled to room temperature and poured into a beaker containing 500 ml crushed ice followed by addition of ammonium solution until the pH was 7. The neutral mixture was then filtered through a 0.22-m polytetrafluoroethylene membrane and filtrate was dialyzed in 1 kDa dialysis bag for 5 days. After purification, the solution was concentrated using rotary evaporation and the carbon dots solution was collected. The synthesized carbon dot is denoted as Coal-T60-CDTs in the subsequent part of the description.

Example 1.2 Fabrication of Carbon Dots from Coal Samples (Tirap-20)

(16) The same experimental procedure as outlined in Example 1.1 was conducted with Tirap-20 coal sample in the same manner. The synthesized carbon dot is denoted as Coal-T20-CDTs.

Example 1.3 Fabrication of Carbon Dots from Coal Samples (Coal-NK)

(17) The same experimental procedure as outlined in Example 1.1 was conducted with Coal-NK in the same manner. The synthesized carbon dot is denoted as Coal-NK-CDTs.

Example 1.4 Fabrication of Carbon Dots from Coal Sample (Coal-NG)

(18) The same experimental procedure as outlined in Example 1.1 was conducted with the Coal-NG in the same manner. The synthesized carbon dot is denoted as Coal-NG-CDTs.

(19) The concentration of the as-synthesized carbon dots was calculated by using Thermal Analyzer and estimated to be about 7.4 mg/ml (Coal-T20-CDTs), 4.4 mg/ml (Coal-NK-CDTs), 4.2 mg/ml (Coal-NG-CDTs), and 2.4 mg/ml (Coal-T60-CDTs).

(20) The microstructure/nanostructure of the synthesized carbon dots was investigated by using transmission electron microscope (TEM/HRTEM). The diameters of these fabricated carbon dots were estimated to be in the range of 1-6 nm, 2-5 nm, 10-30 nm, and 1-4 nm for Coal-T60-CDTs, Coal-T20-CDTs, Coal-NK-CDTs, and Coal-NG-CDTs respectively, which revealed the formation of carbon quantum dots (CQDTS) as well as graphene quantum dots (GQDTS). In some embodiments, the formed carbon quantum dots and graphene quantum dots had a crystal structure. In some embodiments, the formed graphene quantum dots had single layer to multiple layers. In some embodiments, the formed carbon quantum dots and graphene quantum dots included amorphous carbon addends on their edge.

(21) The FTIR analysis of the synthesized carbon dots showed the presence of CC, CO, CO, HC, and OH vibration modes. The intensity of the CO, CO and OH vibrations modes was found to be increased, owing to the introduction of hydrophilic functionalities and consequently, the carbon dots showed high solubility in water.

(22) The Raman spectra of the carbon dots showed mainly two characteristic bands appearing at 1616 cm.sup.1 (G-band) and 1375 cm.sup.1 (D-band) for Coal-T60-CDTs; 1553 cm.sup.1(G-band) and 1381 cm.sup.1 (D-band) for Coal-T20-CDTs; 1553 cm.sup.1 (G-band) and 1382 cm.sup.1 (D-band) for Coal-NK-CDTs; 1595 cm.sup.1 (G-band) and 1387 cm.sup.1 (D-band) for Coal-NG-CDTs. The G-band originated from the vibration of the sp.sup.2-hybridized carbon framework in the 2D hexagonal lattice of graphite cluster and D-band originated from a lattice defect including the sp.sup.a hybridized carbon. The G-bands correspond to the first-order scattering of the E.sub.2g stretching mode of graphite. The D-band is due to the residual ill-organized graphite.

(23) The photo-physical properties of the carbon dots were investigated by using ultraviolet (UV-vis) spectroscopy, FL spectroscopy, and time-resolved single-photon counting spectroscopy. The fabricated carbon dot was excited at 300 nm and the corresponding ultraviolet absorption was observed. The absorption bands appeared at around 220-300 nm and were due to the excitation of pi-electrons (.fwdarw.*) of the aromatic n system. The molar absorption coefficient () of the blue-emitting Carbon Dots was estimated to be about 5662.50 M.sup.1cm.sup.1 (Coal-NG-CDTs), 18716.25 M.sup.1cm.sup.1 (Coal-NK-CDTs), 10500 M.sup.1cm.sup.1 (Coal-T60-CDTs), and 22500 M.sup.1cm.sup.1 (Coal-T20-CDTs). The emission maximums of carbon dots solution were at around 460 nm, corresponding to the blue fluorescence. The fluorescence blue colour of the synthesized carbon dots was revealed at 365 nm under UV-lamp.

(24) The time-resolved single-photon counting spectroscopy of the synthesized carbon dots was observed at neutral pH. The corresponding FL life time (), calculated by fitting to exponential using iterative reconvolution, are summarized in Table 2. The observed .sub.1 (<0.8 ns, <1.00 ns, <0.9 ns, and <0.7 ns) for Coal-T60-CDTs, Coal-T20-CDTs, Coal-NK-CDTs, and Coal-NG-CDTs respectively are thought to be due to the photoluminescence decay of the aggregated state. The life times .sub.3 (>10 ns, >9 ns, >8 ns, and >8 ns) for Coal-T60-CDTs, Coal-T20-CDTs, Coal-NK-CDTs, and Coal-NG-CDTs respectively were longer in comparison to the ones reported earlier, which accounts for the higher PL emission.

(25) TABLE-US-00002 TABLE 2 Life time () calculated from the time-resolved decay profile of the synthesized carbon dots (CDTS). Avg Samples .sub.1 (ns) f.sub.1 .sub.2 (ns) f.sub.2 .sub.3 (ns) f.sub.3 Coal-T60-CDTs 0.791 15.84 2.747 19.51 10.139 64.65 4.55 0.088 0.028 8.7e4 Coal-T20-CDTs 0.992 19.49 3.520 31.60 9.982 48.91 4.83 0.069 0.017 0.002 Coal-NK-CDTs 0.870 23.02 3.340 34.00 8.873 42.98 4.36 0.067 0.021 0.004 Coal-NG-CDTs 0.626 16.37 2.453 39.32 8.105 44.30 3.72 0.156 0.018 0.002
Quantum Yield of the Carbon Dots:

(26) The quantum yield of the as-fabricated coal-derived carbon dots were calculated with the following formula:
=rI/IrAr/A/r
wherein, is the relative quantum yield with respect to the reference/standard. In the present invention, 0.1 mg/L Quinine Sulfate in 0.1M H.sub.2SO.sub.4 solution was used as standards. I is the measured integrated emission intensity, is the refractive index of the solvent, and A is the optical density (absorbance). The subscript R refers to standard index of the Quinine Sulfate.

(27) In the present invention, the FL quantum yield of the as-fabricated carbon dots was calculated to be about 3% (Coal-T20-CDTs), 4% (Coal-NK-CDTs), 8% (Coal-T60-CDTs), and 14% (Coal-NG-CDTs). These are quite higher than the other CDTs reported in prior art. In some embodiments, the quantum yield may vary depending upon the carbon source.

Example 1.5

(28) Antimicrobial, Antifungal, and Cytotoxicity Test of the Fabricated Carbon Dots

(29) As the proposed method for the fabrication of carbon dots from coals completely avoids the use of toxic materials and reagents, the Antimicrobial, Antifungal, and Cytotoxicity test for their biocompatibility and bio-labelling application were also investigated.

(30) Antimicrobial and Antifungal Test:

(31) The antimicrobial activity of the as-fabricated carbon dots was tested against five bacterial species [gram negative: Pseudomonas aeruginosa (MTCC2453), Escherichia coli (MTCC739), and gram positive: Mycobacterium abscessus (ATCC19977), Staphylococcus aureus (MTCC96) and Bacillus subtilis (MTCC441)] and two fungal species [Candida albicans (MTCC3017) and Fusarium oxysporum(NCIM1281)] respectively. For the activity assessment, 100 l of each of the bacterial and fungal culture was inoculated in Nutrient agar plates and potato dextrose agar plate using spread plate method, respectively. In each plate, four 6-mm wells were prepared using sterilized cork-borer and 50 l of each test sample was inoculated in it. The as prepared bacterial and fungal plates were then incubated at 35 C. for 24 h and 28 C. for 5 days respectively. Finally, the inhibition zone (mm) was recorded. It was observed that 50 l of the as-fabricated carbon dots do not inhibit the growth of any bacterial and fungal strains.

(32) Cytotoxicity Test:

(33) The commercially available cell Lines namely, L6 (Rat muscle cell line), HeLa (Human cervical cancer cell line), PC3 (Human prostate cancer cell line) and MDAMB 231 (Human breast adenocarcinoma cell line) procured from NCCS, Pune and cultured in respective complete media (DMEM for L6 and MDA-MB-231, MEM for HeLa, and Ham's F12k for PC3) supplemented with 10% Fetus Bovine Serum (FBS), 10% Penstrep, 1% Gentamycin were incubated under standard conditions in 37 C. humidified 5% CO.sub.2 atmosphere. After reaching confluence (110.sup.6 cells per ml) cells were seeded in tissue culture grade 4-well Millicell EZ Slide (Millipore) in complete medium and incubated. After 24 hrs, the complete medium was replaced with FBS free medium and incubated overnight. The cells were then treated with planned amount of the as-fabricated carbon dots into each well and incubated for 3 h. Cell maintained in sample free medium served as control. After the treatment, the cells were washed 3 times with phosphate buffer to remove any particles not taken up by the cell. The slide was removed from the holder and mounted with fluoroshield (Sigma). The mounted slide was checked under a fluorescent microscope (Motic AE31) and image of the cells captured with excitation at 490 and 557 nm.

(34) After treatment, all the cells were attached with the slide and no change in the morphology of the cells were detected against control. Cell Viability (MTT) Assay of as-fabricated carbon dots in HeLa cell line induced moderate cell death in a dose-dependent manner.

(35) After 3 h incubation, the survival rate was higher than 70% even if the concentration of the as-fabricated carbon dots was increased up to 24 g/mL (Coal-T60-CDTs), 44 g/mL (Coal-NK-CDTs), 42 g/mL (Coal-NG-CDTs), and 74 g/mL (Coal-T20-CDTs), which indicates the fairly low toxicity of the as-fabricated carbon dots. Next, cell imaging was performed on a fluorescence microscope after incubating L6 cell line with the planned amount of as-fabricated carbon dots for 3 h. The bright field image showed that the treated cell retained their original fusiform morphology, which also confirmed the low toxicity of the as-fabricated carbon dots. FITC image, TRITC image, and merged image showed that L6 cells labelled by as-fabricated carbon dots shine under UV-radiation which further indicated that the developed carbon dots could be used as a promising material for optical-imaging/bio-imaging.

(36) In summary, the present invention concerning coal-derived blue fluorescence emitting carbon dots (CDTs) from low-quality Tertiary Indian high sulphur coals, compared to the existing prior arts, have significant molar absorption coefficient, higher FL life time, and higher Quantum Yield () in the range of 5662-22500 M.sup.1 cm.sup.1, 8-10 ns, and 3-14% respectively. Furthermore, the non-toxicity of the coal-derived CDTs to bacterial and fungal strains; and also anticytotoxic activity against human cells was observed.

(37) TABLE-US-00003 TABLE 3 Summary of the characteristics of as-synthesized blue-emitting carbon dots from Indian coals molar absorption Con- coefficient FLlife Quantum centration (M.sup.1cm.sup.1) time Yield Carbon dots (mg/mL) 240 nm) ()(ns) () (%) Coal-T20-CDTs 7.4 22500.00 >9 3 Coal-NK-CDTs 4.4 18716.00 >8 4 Coal-NG-CDTs 4.2 5662.00 >8 14 Coal-T60-CDTs 2.4 10500.00 >10 8

ADVANTAGES OF THE INVENTION

(38) Typical blue-emitting carbon dots (as summarized above) can be produced by a simple and environmentally benign method. The low-quality Indian coal could be converted into highly value-added product like carbon dots (CDTs). Hydrogen peroxide is used as oxidizing agent instead of mineral acid solution (conc. H.sub.2SO.sub.4 and HNO.sub.3) as reported elsewhere, which is highly explosive and difficult to handle during large scale production. The method is less time consuming than the reported methods in the art. Neutralization step is simple. Large volume of water is not required for neutralization step as reported in the art.